An important factor for the correct operation of the majority of the refrigeration compressors is the adequate lubrication of the components that have a relative movement in relation to each other. The lubrication is obtained by pumping lubricant oil provided in an oil sump which is defined in the interior of a lower portion of a generally hermetic shell. This oil is pumped until it reaches the compressor parts presenting relative movement, wherefrom said oil returns, for example, by gravity, to the oil sump.
In some known constructions, the compressor comprises a generally vertical crankshaft carrying a lubricant oil pump, which conducts said oil to the compressor parts to be lubricated, using the rotation of said crankshaft. In these constructions, the oil is pumped from the oil sump by spinning and mechanical dragging.
Technology has been increasingly improving the performance of the refrigeration compressors, and one of the forms to obtain such improvements is through the modulation of the refrigeration capacity of the compressor, upon operation thereof in the refrigeration system to which it is coupled, which permits to reduce the operating rotation of said compressor, when the thermal load is reduced. This procedure is carried out with variable speed compressors (VCC), which permit obtaining considerable performance gains of the refrigeration system. Nevertheless, for the good operation of the compressor at low rotations, further improvements in some constructive aspects of the compressor are still required. One of these constructive aspects refers to the pumping of oil for lubricating the components with relative movement, particularly the bearings. The most employed concept for oil pumping in compressors is based on the centrifugal effect to carry out the pumping. The centrifugal effect uses the pump rotation speed to generate a centrifugal force in the oil. In low rotation operations, this centrifugal effect is impaired, it being necessary to develop other pumping principles in order to comply with the lubrication demand.
There are known some prior art solutions for oil pumping in variable speed compressors. In these constructions (WO93/22557, U.S. Pat. No. 6,450,785), the crankshaft inferiorly carries a pump shaft provided with superficial channels and which is internally disposed in a tubular sleeve, one of the parts of pump shaft and tubular sleeve being rotatively stationary in relation to the other part, so as to provide the dragging effect on the oil being suctioned by centrifugal force caused by rotation of the motor.
The solution disclosed in WO93/22557 presents the pump shaft externally provided with helical grooves and affixed to the crankshaft, in order to rotate therewith, the tubular sleeve being attached to the electric motor stator, by a fixation rod, said tubular sleeve being mounted around the pump shaft with a radial gap.
The solution disclosed in U.S. Pat. No. 6,450,785 presents the pump shaft externally provided with helical grooves on its outer surface and inferiorly attached to the electric motor stator, in order to remain stationary, while the tubular sleeve rotates together with the shaft and the rotor of the electric motor.
The solution object of Brazilian Patent Application PI0604908-7 presents an oil pump in which the tubular sleeve is provided with helical grooves on its inner surface and affixed to the rotor-crankshaft assembly, the pump shaft being attached to one of the parts of stator and shell.
This oil pump construction results in a higher pumping efficiency, allowing an efficient pumping mainly at low rotations. The pumping principle of this construction permits the compressors to operate with capacity modulation at extremely low rotations.
For a better pumping of oil from the oil sump, it is desirable that the oil elevation channel, defined by the helical groove in the tubular sleeve of the oil pump, is made with the greatest possible diameter, said helical groove being provided internally to the tubular sleeve, which rotates so that the oil pumped from the oil sump, by centrifugal force, is pushed to the bottom of the helical groove and dragged upwards. Since the tubular sleeve of the oil pump rotates with full compression of the centrifugal force, the oil ascends through the helical groove without escaping therefrom, as the centrifugal force pushes the oil to the bottom of the channel and the side walls of said helical groove do not allow the oil to descend gravitationally. This oil seated on the lower part of the helical development of the helical groove is progressively ascendingly dragged. It is always desirable to have the channel provided on the inner surface of the tubular sleeve. However, the machining of the helical groove in a tubular sleeve made of metallic material is extremely difficult, expensive and complex. Thus, it is desirable that the tubular sleeve is made in plastic material, already containing the inner helical groove.
Nevertheless, affixing the tubular sleeve made of plastic material directly in the interior of a lower tubular portion of the crankshaft or of an axial hole of the rotor presents a serious inconvenience, resulting from the fact that the plastic material has its dimensional characteristics altered with time, mainly when submitted to the operational temperature conditions in the interior of the compressor shell. The fixations which use mechanical interference by friction or by threading do not guarantee a reliable, strong and correct retention of the plastic tubular sleeve during the desired useful life of the compressor, allowing the occurrence of misalignments, faster wear of the involved pieces and insufficient oil pumping to promote the degree of lubrication required by the compressor project.